The present invention relates to polyurethane adhesives which are at least partially crystalline at room temperature.
Thermoplastic polyurethane adhesives which acquire their strength from the increase in viscosity and/or crystallization upon cooling are well known in the art. These polymer adhesives must be substantially linear in structure to enable them to be worked up and therefore have little adhesive strength at elevated temperatures (see Vieweg/Hochtlen, Kunststoffhandbuch, Volume 7, Polyurethane, Carl-Hanser Verlag, Munich 1966). In such hot melt adhesives it is necessary to accept low heat resistance under load for the sake of obtaining a low processing temperature. If, on the other hand, it is desired to obtain bonds which have high stability at high temperatures, conventional hot melt adhesives must be employed at very high application temperatures, which is not always possible.
One solution to this problem is provided by reactive polyurethane systems. The polymer, which has a low viscosity and low molecular weight during processing, undergoes a chemical reaction during processing, whereby its molecular weight and bond strength are increased. Reactive polyurethane systems, may be formulated as either two-component or one-component systems. Two-component polyurethane systems consist of an isocyanate-containing component and an isocyanate reactive component. These two components must be mixed together before processing. This gives rise to adhesives which have a low processing temperature and excellent thermal stability. One disadvantage of these adhesives is the costly method of processing required and the low storage stability and hence short working life of the adhesive mixture once it is ready for use.
The disadvantage of a limited shelf life does not occur to the same extent in true one-component polyurethane systems.
German Auslegeschrift 3,543,120 describes a polyurethane hot melt adhesive which has a low processing viscosity, a long processing time and good thermal stability due to containing liquid polyols. These characteristics are obtained by using so-called masked isocyanates. The hot melt adhesives are prepared as two-component adhesives by mixing a polyisocyanate and a polyol mixture containing the masking agent immediately before use so that the adhesive is formed after it has been applied to the substrate which is to be bonded. This procedure requires considerable technical skill in the mixing technique for the user and is therefore costly. Curing of these adhesives requires high temperatures.
Polyurethane hot melt adhesives which are worked up as one-component systems and are also cross-linked by means of masked isocyanates are described in European Patent 248,658. This reference describes hot melt adhesives based on high melting polyesters which contain aromatic dicarboxylic acids as their main component. The high temperatures required for working up (T&gt;100.degree. to 125.degree. C.) and curing (T=180.degree. C.) these adhesives severely restricts their usefulness, e.g. for bonding plastics. Further, such hot melt adhesives have the disadvantage of softening again at temperatures above 110.degree. C.
U.S. Pat. No. 4,722,969 describes one-component reactive systems which are to be processed at low temperatures but due to the high degree of branching required, the adhesives do not have the degree of flexibility required for flexible substrates, and due to the large amount of catalyst required for achieving the necessary rapid curing, the products have a storage stability of only two months at room temperature.
Masked isocyanates as components of an adhesive system are also described in U.S. Pat. No. 4,798,879. This patent describes a two-component adhesive system which sets rapidly at room temperature and which consists of a prepolymer containing masked isocyanate groups and primary amines as hardeners. It is all the more surprising that according to the invention described below, storage stable one-component systems may be obtained from prepolymers containing masked isocyanate groups and aminic or alcoholic hardeners.
It was an object of the present invention to provide adhesive compounds with good storage stability at elevated temperatures, low melt viscosity and rapid increase in viscosity at a relatively low curing temperature and rapid hardening.